There are other possibilities for getting diploids from tetraploids (or haploids from diploids more generally).
If a tetraploid plant is irradiated (with x-rays, ultraviolet light, etc) at an appropriate dose at the appropriate time when the pollen is developing then the chromosomes can be effectively 'destroyed' but the pollen maintains its viability. If that irradiated pollen is used to pollinate a tetraploid then it is possible for some of the ovules to develop into diploids (generally scientifically into haploids).
This article provides an introduction to various methods of producing haploids.
http://www.intechopen.com/book...
